In traction elevator systems the elevator cab is connected to a counterweight unit by hoist ropes and a sheave or pulley system. The cab and the counterweight unit are each typically mounted between a pair of vertically extended guide rails and are arranged to ride along the rails upon rollers or guide shoes. The guide rails are generally T-shaped sections that are normally erected in sixteen foot lengths and are attached to the building housing the elevator system by support brackets.
In assembly, the base flange of the T-shaped vertical sections are attached to support brackets by clips so that the blade or web section of each rail points inwardly towards the cab or the counterweight unit as the case may be. The guide roller or shoes ride along the blades along with the safeties which are designed to apply a sufficient frictionally holding force against the rails to bring the cab to a rapid and safe stop in the event an overspeed condition is sensed. As can be seen, the rail sections must be precisely aligned in assembly because they determine the positioning of the elevator in the hoistway and the related positioning of much of the operating equipment.
The clips used to mount the rail sections to the support brackets must permit the rail sections to shift or move longitudinally in the event the building housing the elevator system settles or the support brackets and/or the building to which the brackets are attached deform non-uniformly due to thermal stresses. The requirements for rail clips are thus not necessarily mutually compatible. The clips must, on one hand, provide sufficient hold force to support the rail weight plus the dynamic load produced by a car moving over the rail and the car during safety engagements. On the other hand, the clips must allow the rail to slide vertically in the event the building housing the elevator system settles or there is a difference in thermal expansion between the building and the rails. This requires that the clamping force exerted by the clips must be within a certain predetermined range. A change in the lateral distance between the support bracket and the contact point, due to rail tolerances or a rust build-up under the rail, will effect preload force moving the force outside the predetermined range. This can, in turn, cause the rail to buckle, thus adversely effecting the smoothness of the ride as the cab moves over the rails. Accordingly, the clips must be soft enough under normal conditions to accommodate the change in the lateral distance between the support bracket and the contact point without significant change in a clamping force, yet stiff enough to resist lateral displacement of the rails under excessive lateral displacement.
The disadvantage of a soft clip is that although they accommodate a rust build up under the rail or variation in the rail tolerances without substantial change in clamping force, it generally cannot meet the code requirements for seismic applications. On the other hand, for rigid seismic clips, any such change in lateral displacement can dramatically increase the holding force that the clip exerts on the rail and thus adversely effect the rail's ability to slide longitudinally.